The present invention relates to dispersive optical devices. More specifically, but not exclusively, the invention relates to an improved arrayed waveguide grating (AWG) device.
In order to meet the ever-increasing demand for transmission bandwidth in communication networks, operators are investing heavily in the development of techniques for Dense Wavelength Division Multiplexing (DWDM). DWDM employs many closely spaced carrier wavelengths, multiplexed together onto a single waveguide such as an optical fibre. The carrier wavelengths are spaced apart by as little as 50GHz in a spacing arrangement designed for an ITU (International Telecommunications Union) channel xe2x80x9cgridxe2x80x9d. Each carrier wavelength may be modulated to provide a respective data transmission channel. By using many channels, the data rate of each channel can be kept down to a manageable level.
Clearly, to utilize this available bandwidth it is necessary to be able to separate, or demultiplex, each channel at a receiver. New optical components for doing this have been designed for this purpose, one of these being the Arrayed Waveguide Grating (AWG). An AWG is a planar structure comprising a number of array waveguides which together act like a diffraction grating in a spectrometer. AWGs can be used as multiplexers or demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. A typical AWG mux/demux 1 is illustrated in FIGS. 1 and 2 and comprises a substrate or xe2x80x9cdiexe2x80x9d 1 having provided thereon at least one single-mode input waveguide 2 for a multiplexed input signal, two slab couplers 3,4 (also sometimes referred to as xe2x80x9cstar couplersxe2x80x9d) connected to either end of an arrayed waveguide grating 5 consisting of an array of transmission waveguides 8, only some of which are shown, and a plurality of single mode output waveguides 10 (only some shown) for outputting respective wavelength channel outputs from the second (output) slab coupler 4 to the edge 12 of the die 1. In generally known manner, there is a constant predetermined optical path length difference between the lengths of adjacent waveguides 8 in the array which determines the position of the wavelength output channels on the output face of the second slab coupler 4. The construction and operation of such AWGs is well known in the art. See for example, xe2x80x9cPHASAR-based WDM-Devices: Principles, Design and Applicationsxe2x80x9d, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol.2, No.2, June 1996. In some cases, there are no input waveguides: instead, the first slab coupler 3 is arranged at the edge of the die 1, so that an input signal can be launched directly into the slab.
One problem with AWGs is that signal coupling between the output waveguides often occurs. This tends to increase crosstalk between adjacent channels of the AWG which is detrimental to performance.
Japanese unexamined patent application, JP2000171648, describes one approach for minimizing adjacent crosstalk in an AWG. This involves decreasing the spot size of the single mode input and output waveguides where they are connected to the slab couplers. This can be done by reducing the width of all the waveguides or by changing the refractive index of all the waveguides. Spot size is however only easily definable for single mode waveguides. Moreover, decreasing the width of the waveguides at the coupler tends to decrease the usable bandwidth of the signal outputs from the respective wavelength channels. Another embodiment is also shown in which the spot size is so reduced only in alternate ones of the input/output waveguides. A significant disadvantage of this latter solution, where the reduced spot size is achieved by reducing the width of the waveguides, is that alternate waveguides are thus of different widths at the slab coupler. The waveguide width at the first and second slab couplers affects the passband and bandwidth of the output wavelength channels and so this solution has the disadvantage that the output wavelength channels are not of uniform passband (i.e. power output response) across all the output channels.
It is an aim of the present invention to avoid or minimize one or more of the foregoing disadvantages.
According to a first aspect of the invention there is provided an arrayed waveguide grating (AWG) device comprising: first and second slab couplers; a plurality of array waveguides optically coupled between the first and second slab couplers and having predetermined optical path length differences therebetween; and a plurality of output waveguides optically coupled at first ends thereof to an output side of the second slab coupler, for outputting different wavelength channel outputs therefrom; wherein the output waveguides are multi-mode waveguides; at least one of the output waveguides, along at least a portion of its length, is of different width to at least one adjacent one of the output waveguides along an adjacent portion of the length thereof; and all the output waveguides are of substantially equal width at said first ends thereof where they are coupled to the second slab coupler.
The AWG according to the invention has the advantage of reduced coupling between the output waveguides due to at least some adjacent ones of the output waveguides being of different widths along adjacent portions of their lengths, while the passband of each output wavelength channel is substantially the same by virtue of the output waveguides all being of substantially the same width where they are optically coupled to the second slab coupler.
In the preferred embodiment, said at least one of the output waveguides is tapered in width along an initial portion thereof, from said first end thereof.
Each output waveguide is preferably a double mode waveguide. The widths of the output waveguides may alternate through a series of N different widths (e.g. 3 different widths), between corresponding first and second portions of the lengths of the output waveguides, whereby every Nth one (e.g every third one ) of the output waveguides is of the same width along a corresponding portion of its length. In the preferred embodiment, adjacent ones of the output waveguides are of different widths along at least a portion of their lengths, preferably along at least an initial portion proximal to the second slab coupler. Some of the output waveguides, for example alternate ones, may be of substantially uniform width along their entire lengths, between first and second ends thereof, with no tapering. In a further possibility, each output waveguide is tapered at first end portions thereof and second portions along the length thereof.
According to a second aspect of the invention we provide a power monitor comprising: an arrayed waveguide grating (AWG) as above-described; and detector means for detecting the different wavelength channel outputs at output ends of the output waveguides.
According to a further aspect of the invention there is provided an arrayed waveguide grating (AWG) device comprising: first and second slab couplers; a plurality of array waveguides optically coupled between the first and second slab couplers and having predetermined optical path length differences therebetween; and a plurality of output waveguides optically coupled at first ends thereof to an output side of the second slab coupler, for outputting different wavelength channel outputs therefrom; wherein the output waveguides are multi-mode waveguides; at least one of the output waveguides, along at least a portion of its length, is of different width to at least one adjacent one of the output waveguides along an adjacent portion of the length thereof; said at least one of the output waveguides is tapered in width along an initial portion thereof, from said first end thereof; and the widths of the output waveguides at said first ends thereof where they are optically coupled to the second slab coupler are designed so as to achieve a substantially uniform passband over all the output wavelength channels of the device.